The present invention pertains to continuous reactor processes and in particular the use of such reactors to effect reaction between a liquid and a reactant gas.
In the manufacture of precipitated calcium carbonate it is conventional to use a batch reactor, a continuously stirred tank reactor (CSTR) or a pipe-line-type plug flow (PF) reactor to contact a liquid slurry of water and calcium hydroxide with carbon dioxide in order to synthesize precipitated calcium carbonate having particular characteristics.
Continuous stirred tank reactors rely upon a mechanical agitator and the introduction of the reactant gas directly into the liquid to achieve the desired reaction. The continuous stirred tank reactor is operated at predetermined temperatures, pressures and agitation rates in accord with the product being produced by the contact of a liquid with a reactant gas. Continuous stirred tank reactors are generally limited in size. In order to achieve increased system throughput or economics of scale, multiple reactors must be employed.
The plug flow reactor is generally a long tubular shape reactor filled with the liquid which is generally moving in a straight line direction into which the reactant gas is introduced. Plug flow reactors are generally expensive since they require a long pipe line and the use of a high purity gases in certain applications. Two reasons for using high purity gas are, to avoid slugging and to enable the use of smaller size pipe.
Numerous techniques have been used to produce precipitated calcium carbonate having a controlled particle size for use in various applications and in particular the treatment of papers.
U.S. Pat. No. 2,538,802 discloses and claims a continuous process for producing precipitated calcium carbonate having a desired particle size range using a two-stage dual carbonator system. [Patentees give details of other reactors that were available at the time, i.e. prior to 1951.]
U.S. Pat. No. 3,150,926 discloses and claims a continuous process for producing precipitated calcium carbonate using an elongated reactor having dual screw type conveyors to move the slurry from the entry end to a discharge end of the reactor. Paddles and longitudinal blades are used to move the materials through the reactor in what patentees describe as a flow pattern xe2x80x9clikened it to a rock and curve-bound stream wherein the stream flow is basically in one direction although the obstacles and curves create back flows, eddys and swirls which slow the rate of flow while keeping the entire stream in a constant state of agitation.xe2x80x9d Patentees also described the action as that of a xe2x80x9cmechanically fluidized bed.xe2x80x9d The reactor is enclosed and carbon dioxide is introduced through the bottom of the reactor in what is called the carbonation zone.
U.S. Pat. No. 4,133,894 discloses and claims a multi-step, multi-vessel process for preparing precipitated calcium carbonate having less that a 0.1 xcexcm particle size. Various processing parameters are disclosed.
U.S. Pat. No. 4,888,160 discloses and claims using a stirred tank reactor for preparing various precipitated calcium carbonate products. The Patent discloses control of various parameters, e.g. pH, composition of the slurry, temperature, reacting gas purity, and the use of inhibitors to achieve the desired particle shape.
Other types of reactors which show varying types of flow to introduce a gaseous reactant into a slurry are exemplified by U.S. Pat. Nos. 2,000,953; 2,704,206; 3,045,984; 3,417,967; 3,856,270; 4,313,680; and 4,514,095. All of the foregoing reactors use complex mechanisms to provide a motion or direction change to a slurry moving through the reactor to enhance gas-liquid contact.
There is a need to provide for both improved processes for gas liquid contacting and improved apparatus that can be fabricated easily and economically to carry out such processes.
The present invention pertains to a method and apparatus for improving contact between a liquid and a gas, either or both of which may be a reactant. The process of the present invention involves causing the liquid to move in a serpentine path through a reactor so that the serpentine path causes the liquid to move both laterally and vertically as the liquid proceeds from one station, section, stage, zone, or chamber to another in a novel reactor. As the liquid moves in the serpentine path gas is introduced below the surface of the moving liquid at, at least one, but preferably many locations in each zone. The reactor according to the present invention is designed to effect movement of the liquid in the horizontal and vertical serpentine motion (tortuous flow) through discrete chambers in the reactor. Gas can be introduced into the liquid in any one or all of the chambers.
The number of chambers in a reactor can be constructed in a single line or in banks of rows arrayed side-by-side and reactors can be ganged together in various lateral, or nested arrays in order to achieve the required gas liquid contact. In point of fact the chambers can be arranged in any configuration to accommodate the constraints of a particular plant layout, as long as the flow path is as described between the chambers. Thus a reactor according to the invention can have any number chambers arranged in any number of rows inside a given reactor. The reactor can be multiple reactors or modules connected in series to achieve an overall reactor of any required length that defines a continuous flow path.
Therefore, in one aspect the present invention is a continuous gas-liquid contact reactor comprising in combination; an elongated housing having the general shape of a four sided polygon, the housing adapted to contain a bath of liquid, a plurality of individual chambers disposed within the tank, the chambers arranged to permit the liquid to flow sequentially from a first chamber to a last chamber, means to introduce the liquid into the first chamber and withdraw liquid from the last chamber, means in the housing to direct the liquid from a point of entry in each chamber, being one of at a top corner or a diagonally opposed bottom corner, in a general direction to point of entry into the succeeding chamber which is diametrically opposed to the point of ending from the previous chamber, and means to introduce a gas, optionally a reactant gas, into one or more of the chambers below the level of liquid flowing through the chamber. When the gas is not a reactant gas, the liquid is typically composed of merge streams of reactants. In another aspect the present invention relates to a method for enhancing contact between a liquid and a gas, e.g. a reactant gas, comprising the steps of; moving the liquid along a confined path from a point of entry to a point of exit in a generally elongated vessel, the liquid caused to move in a generally serpentine path through a plurality of stages or chambers in the vessel, the serpentine path being defined as causing the liquid to move laterally and alternately from top to bottom or from the bottom to the top in each of the chambers, and introducing the gas into the liquid in at least one of the chambers through which the liquid is moving.
The present invention includes a further optional method step of recycling gas, such as the unreacted collected reactant gas back to the liquid or some other part of an overall process. For example in the manufacture of precipitated calcium carbonate, carbon dioxide escaping from the bath, where it reacts with the calcium hydroxide in the water, could be collected and recycled to the compressor, blower, or fan used to introduce fresh carbon dioxide into the process.
In still another aspect the present invention is a precipitated calcium carbonate having any of the known crystalline structures, for example, a calcitic or aragonitic crystalline structure or mixtures of both calcitic and aragonitic precipitated calcium carbonate, made by reacting a liquid containing calcium hydroxide and water with a reactant gas containing carbon dioxide produced by; moving the liquid along a confined path from a point of entry to a point of exit in a generally elongated vessel; the liquid caused to move in a generally serpentine path through a plurality of stages or chambers in the vessel, the serpentine path being defined as causing the liquid to move laterally and vertically in each of the chambers, and, introducing the reactant gas into the liquid in at least one of the chambers through which the liquid is moving.
The present invention also pertains to a method of producing a precipitated calcium carbonate with a controlled crystalline structure by contacting a liquid containing calcium hydroxide and water with a reactant gas containing carbon dioxide comprising the steps of; moving the liquid along a confined path from a point of entry to a point of exit in a generally elongated vessel, the liquid caused to move in a generally serpentine path through a plurality of stages or chambers in the vessel the serpentine path being defined as causing the liquid to move laterally and vertically in each of the chambers, and introducing the reactant gas into the liquid in at least one of the chambers through which the liquid is moving.